(1) Field of the Invention
The present invention relates to a head driving device to be used for a flexible disk, a hard disk, or an optical disk which record or reproduce data, and especially to a head driving device provided with a linear motor.
(3) Related Art
Recently, A4 size or smaller portable personal computers and word processors have been developed. These devices are expected to be thin and light in weight, and to demand low power consumption to be used in a car or train, and also to be inexpensive.
These devices are provided with a disk to record and reproduce data, which has a head driving mechanism which allows the head to slide in the radial direction.
A popular head driving mechanism to be used in a disk for a portable PC or word processor is a voice coil type linear motor provided with a yoke, a permanent magnet, and a moving-coil.
A head driving device provided with an unipolar voice coil type linear motor having a single pole magnet is disclosed, for example, in U.S. Pat. No. 4,969,058 and Japanese Laid-open Patent Application No. 2-114845.
A head driving device provided with a multipolar voice coil type linear motor having multipolar magnets is disclosed, for example, in U.S. Pat. No. 4,669,013.
The multipolar voice coil type linear motors cause less concentration of the magnetic polarities than the unipolar voice coil type ones. As a result, magnetic saturation of a stator yoke is reduced. This contributes to the reduction in leakage flux, and as a result, a thin and light weight motor having a low power consumption can be realized.
Furthermore, Japanese Laid-open Patent Application No. 3-270670 discloses a multipolar voice coil type linear motor which can prevent irregular torque by making the dispersion of magnetic flux into a sine wave.
However, in a multipolar voice coil type linear motor, the direction of a magnetic field to be formed in the passage for the coils to move is alternated in accordance with the direction that the coils proceed. Consequently, it is necessary to detect the direction of the magnetic field as the coils proceed and to supply current in accordance with the detected direction. For this reason, the current to supply to the coils is controlled by detecting the magnetic flux by providing hole elements in the vicinity of the coil.
FIG. 1 shows the construction of a multipolar voice coil type linear motor having hole elements disclosed in the Japanese Laid-open Patent Application No. 2-131354.
The linear motor is provided with a pair of multipolar magnets 60 and 61 attached to the yoke 67, and first and second coils 63 and 64 which are movable through the space 62 extending between the multipolar magnets 60 and 61. The coils 63 and 64 have hole elements 65 and 66. In the space 62, the multipolar magnets 60 and 61 form a magnetic field whose direction alternates in accordance with the direction of the movement of the coils 63 and 64.
In such a linear motor, the driving force for the coils 63 and 64 is in proportion to the product between the magnetic flux of the coils 63 and 64 and the current supplied to the coils 63 and 64. Consequently, the coils 63 and 64 can be moved in the same direction by detecting the magnetic flux with the hole elements 65 and 66 and shifting the direction of the current to be supplied in accordance with the direction of the magnetic field.
FIG. 2 is a block diagram showing a driving circuit to drive a multipolar voice coil type linear motor.
The position detection unit 20, which is composed of, for example, an optical encoder and a counter, detects the positions of the coils 63 and 64. The comparison unit 32 calculates the error between the positional instruction 38 which indicates the positions of the coils 63 and 64 and the current position detected by the position detection unit 20.
The compensation unit 33 performs stability compensation or deviation compensation for the positional error calculated by the comparison unit 32, thereby generating a driving force instruction. The multiplication units 34 and 35 multiplies the magnetic flux detected with the hole elements and the driving force instruction generated by the compensation unit 33 in order to generate current instructions to supply to the coils 63 and 64. The current amplifiers 36 and 37 apply the current instruction generated by the multiplication units 34 and 35 to the linear motor 60 by amplifying current. Consequently, the coils 63 and 64 are moved by giving driving force in order to match the position to be detected by the position detection unit 20 and the positional instruction 38. The magnetic head can be located on a desired track by moving it together with the coils 63 and 64 by such a driving circuit.
However, when a multipolar voice coil type linear motor having hole elements is used as a head driving device, there are following problems.
More components are needed because of the provision of hole elements.
Since the outputs from the hole elements are analog, it is impossible to directly perform digital process.
The hole elements have different sensitivity, temperature characteristics, and also they change as time passes. These features directly affect the torque, and consequently, an adjustment for optimizing the driving circuit is needed.
The hole elements are originally used to detect the magnetic field to be formed by a permanent magnet; however, they tend to mistakenly detect the magnetic field to be generated by the coils because they are disposed in the vicinity of the coils. Such a miss operation tends to be caused especially during an access to a track, because a large amount of current is supplied to the coils. In addition, the S/N ratio of the hole elements deteriorates when a positioning operation is controlled, and as a result, the accuracy of the positioning operation is deteriorated.
It is possible to dispose the hole elements away from the coils to solve these problems; however, it prevents the device from being minimized.